You can already just use magic or just use mundane disguises. Model changes are ofc the target, that way you could actually pass for someone else.

When I thought about it briefly I saw a couple of issues : first the number of models is limited and you can get a perfect copy .. secondly you won't have access to your inventory, thirdly such a system would need to change the appearance of the gear used by the model you're changing shapes to. Big hurdles.

And anything done outside a shapechange effect through direct BIC editing would be fairly dangerous in terms of character corruption as far as I can tell.

I'm sure a similar system to Sigil's appearance changer for succubus/doppelganger/dragon humanoid forms could work well with the Disguise self spell. Question is whether Sigil wants to share their work, as their devs seem to look negatively upon BG's devs/admin for some reason.